One of the simplest and least expensive methods of microwave power division and directional coupling is by the direct coupled or branch line type of structure. This method of construction is particularly suitable to a single plane configuration and has the advantage of dc continuity. The simplest of branch line couplers is the two section version shown in FIG. 1a. It consists of a mainline which is coupled to a secondary line by two quarterwave-long sections spaced one quarter wavelength apart. Thus, it has a circumference of approximately one wavelength. The coupling factor is determined by the ratios of the impedances of shunt and series branch arms and it is adjusted to maintain a proper match over frequency. The impedance ratios necessary for proper coupling are widely known. See for example, H. Howe Jr.; "Stripline Circuit Design," Chapter 3, Artech House, Inc.
Some practical applications of branch line couplers include: 1. Power division for image rejection mixers and single sideband modulators; 2. Circuits requiring reflection of mismatches into a terminated fourth port load.
In a typical design, for example, a 3 dB branch line coupler for operation in a 50 Ohm system, the series arms are of 35.4 Ohm characteristic impedance. The shunt arms are of 50 Ohm characteristic impedance. In this case, a signal incident at port 1 divides equally between ports 2 and 3 with a 90 degree phase difference and port 4 remains isolated.
A major problem with branch line couplers arises at high frequencies. Since wavelength is inversely proportional to frequency, branch line couplers designed for high frequency operation will have shorter arm lengths but the arm widths are unchanged. See FIG. 1b. Thus, the length of the line approaches its width such that there is a relatively large junction point which consists of merged and nondistinct lengths and widths for the branch lines. This translates into a large junction discontinuity, severely affecting the microwave performance of the device. Numerous reports have dealt with how to analyze and to compensate the junction discontinuity reactance, but not how to reduce the same. See for example, (1) R. Mehran: "Compensation of microstrip bends and Y-junctions with arbitrary angle," IEEE transactions on Microwave Theory and Techniques, vol. MTT-26, pp. 400-405, June 1978; (2) R. Chadha and K.C. Gupta: "Compensation of Discontinuities in Planar Transmission Lines," IEEE transactions on Microwave Theory and Techniques, vol. MTT-30, pp. 2151-2155, Dec. 1982. A major advance would thus be represented where a localized junction between branch arms could be designed such that there are distinct arm lengths and widths at high frequencies. Gaining such a control over the junction size has important implications in all microwave circuits, particularly in monolithic integrated circuits.